Observations from four gravitational wave detectors, three in the U.S. and one in Europe, have allowed scientists to narrow down the possibilities of how the universe looked in its earliest moments.
A paper published in the journal Nature on August 20 reports new analysis of measurements by the LIGO (Laser Interferometer Gravitational-Wave Observatory) Scientific Collaboration, which includes several scientists from the University of Wisconsin–Milwaukee (UWM).
In particular, Xavier Siemens, UWM assistant professor of physics, and Warren Anderson, associate scientist in UWM’s Center for Gravitation and Cosmology, made significant contributions.
The group’s work has shown how the search for elusive gravitational waves with the ultra-sensitive detector facilities has resulted in new information about the early universe. Gravitational waves are ripples in the fabric of space-time that are produced when massive objects in space move violently. The waves hold secrets about the events that created them and the nature of gravity which cannot be obtained with conventional astronomical tools.
The insight from LIGO data today is a hint of what is to come.
The Big Bang is believed to have created a flood of gravitational waves. These waves still fill the universe today as background “noise,” similar to random ripples on a pond on a windy day.
“With gravitational waves, we’ll ‘see’ the universe when it was much younger – and hotter,” says Siemens.
“Gravitational waves are the only way to directly probe the universe at the moment of its birth; they’re absolutely unique in that regard,” says David Reitze, a professor of physics at the University of Florida and spokesman for the LIGO Scientific Collaboration. Measurements taken between 2005 and 2007 by the detector facilities have set limits on the amount of gravitational waves that could have come from the Big Bang in the gravitational-wave frequency band that LIGO can observe.
Since no waves from the Big Bang have been found yet, says Siemens, LIGO scientists can rule out certain scenarios.
“On the basis of this work, we can eliminate some of the models because at some level, we would have found a signal [indicating gravitational waves] for those to be viable,” he says.
The article in Nature also constrains models of cosmic strings, objects that are proposed to have been left over from the beginning of the universe and subsequently stretched to enormous lengths by the universe’s expansion. “Bursts from cosmic string loops are an example of the kind of events that contribute to the background,” says Siemens. “They’re massive and they emit a lot of gravitational waves. My part was to interpret the results from LIGO in terms of constraints on cosmic string models.”
Anderson served as chair of the review committee for the LIGO Collaboration’s “Stochastic Background Working Group.”
Several scientific papers are cited in the Nature article, including two that were authored by Siemens; Jolien Creighton, UWM associate professor of physics; and Vuk Mandic, assistant professor at the University of Minnesota. One of those papers, published in the journal Physical Review Letters, laid out the procedure for how gravitational-wave analysis results are used to constrain cosmic string models.
The insight from LIGO data today is a hint of what is to come, says Mandic.
Once it goes online in 2014, the upgraded Advanced LIGO, which will use the infrastructure of the LIGO observatories, will allow scientists to detect cataclysmic events such as black-hole and neutron-star collisions at 10-times-greater distances.
The LIGO project, which is funded by the National Science Foundation (NSF), was designed and is operated by Caltech and the Massachusetts Institute of Technology.
Research is carried out by the LIGO Scientific Collaboration, a group of 700 scientists at universities around the United States and in 11 foreign countries.
The LIGO Scientific Collaboration interferometer network includes the LIGO interferometers and the GEO600 interferometer, near Hannover, Germany, and designed and operated by scientists from the Max Planck Institute for Gravitational Physics, along with partners in the United Kingdom funded by the Science and Technology Facilities Council (STFC).
The Virgo Collaboration designed and constructed the 3 km long Virgo interferometer located in Cascina, Italy, funded by the Centre National de la Recherche Scientifique (France) and by the Istituto Nazionale di Fisica Nucleare (Italy). The Virgo Collaboration consists of 200 scientists from five Europe countries and operates the Virgo detector.